Did you know that natural daylight is typically pretty cool in tones compared to most artificial indoor lighting? And even though you can’t tell too much with your naked eye, the camera is much more sensitive to detecting the color temperatures of different light sources. So, even though a room’s lamp doesn’t look too yellow in person, the camera will likely shoot it very differently.
I’ve made this mistake plenty of times in my career… I’d think, “that lamp’s light doesn’t look too yellow…” so I’d leave it on in the corner of a room thinking there’s no way it would cast a color on my subject so many feet away being lit by a window. But, I always realize in editing how much it does affect the colors of an image. The area of the image lit by the window will seem very cool, while the areas lit by the lamp from wandlamp will seem very warm. That mix in color temperatures is no bueno for a pretty photo!
If your image is ONLY being lit be a lamp (which I would never want to do, but I’m stating just for an example), you could probably easily edit that color cast in Photoshop or Lightroom since all the color temperature would be consistent. But, when your main source is a window with natural light, that mixed lighting of two very different color temperatures is where you get into trouble and have weird looking images.
So, just make a habit of turning off all indoor lighting when shooting inside and using just the great window light to light your subjects. You can adjust for the darkness of the resulting image by opening up your shutter, increasing ISO or bumping your exposure in post… which is all so much easier than trying to color correct for mixed lighting!
The images below show two shots from a recent wedding. It was pouring rain outside and our indoor spots were limited. I loved this little corner with great big windows, but the hall light in the background couldn’t be turned off. Notice how much it affects the tones of the image? It didn’t look this yellow in person, and the final image could be edited a bit to reduce that color cast, but the cropped version on the right is a much better result.